Ultrafiltration membranes are typically used in pressure-driven filtration processes. Viral removal membrane filters are increasingly used in the biotechnology industry to provide the needed safety of the manufactured therapeutic products. These filters are meant to retain a high proportion of viruses that may be present in a feed containing the therapeutic product, while the product flows through the membrane.
Ultrafiltration (UF) membranes are primarily used to concentrate or diafilter soluble macromolecules such as proteins, DNA, viruses, starches and natural or synthetic polymers. In the vast majority of applications, ultrafiltration is carried out in the tangential flow filtration (TFF) mode, where the feed solution is passed across the membrane surface and the molecules which are smaller than the pore size of the membrane pass through (filtrate) and the rest (retentate) remains on the upstream side of the membrane. As fluid passes through the membrane, there is a need to recycle or add to the retentate flow in order to maintain an efficient TFF operation. One advantage of using a TFF approach is that because the fluid constantly sweeps across the face of the membrane, it tends to reduce fouling and polarization of the solutes at and near the membrane surface leading to longer life of the membrane.
Ultrafiltration membranes are generally skinned asymmetric membranes, made for the most part on a support, which often remains a permanent part of the membrane structure. The support can be a non-woven fabric, or a preformed membrane. UF membranes are made by immersion casting methods and are skinned and asymmetric. The initial commercial applications were related to protein concentration and membranes were rated by the molecular weight of the protein that they would retain, i.e. the molecular weight cutoff rating of the membrane (MWCO).
While ultrafiltration membrane ratings based on testing with proteins is still performed, a common method uses non-protein macromolecules having a narrow molecular weight distribution, such as polysaccharides (Dextrans) or polyethylene glycols (see for example, “A rejection profile test for ultrafiltration membranes and devices”, Biotechnology 9 (1991) 941-943).                a) Ultrafiltration membrane production methods by immersion casting are well known. A concise discussion is given in “Microfiltration and Ultrafiltration: Principles and Applications”; Marcel Dekker (1996); L. J. Zeman and A. J. Zydney, eds. An exemplary production method is described to consist of the following steps:                    a) preparation of a specific and well controlled polymer solution;                        b) casting the polymer solution in the form of a thin film onto a substrate;        c) coagulating the resulting film of the polymer solution in a non-solvent; and        d) optionally drying the ultrafiltration membrane.        
Controlling pore size in ultrafiltration membranes is generally not straightforward. Not only the solid content of the casting solution has an impact on membrane porosity and pore size but also the relative rates at which non-solvent enters and solvent leaves the casting solution. If the non-solvent enters the film before the solvent leaves, the polymer precipitates around a larger volume of solvent (which acts as a pore former) resulting in high porosity and large pore size UF membrane. The opposite is true if the solvent leaves the film faster than non-solvent enters and the resulting UF membrane has lower porosity and smaller pores. Additives to the casting solution or non-solvent bath as well as temperature adjustment to both are often employed to control the relative rate of non-solvent entry and solvent removal from the cast film.
Agarose is a natural polysaccharide that has been used extensively to produce porous beads. These beads find numerous applications in chromatographic separations. The earliest art describing the formation of agarose beads (for chromatographic applications) used warm, non-aqueous solvents in which the agarose was emulsified before gel formation by cooling. See, for example, Hjerten, S. Biochim. Biophys. Acta 1964, 79:393-398; and Bengtsson et al., S. Biochim. Biophys. Acta 1964, 79:399. Another method for agarose bead formation, as disclosed in U.S. Pat. No. 4,647,536 is dropping an agarose emulsion into a cooled oil. Such a method is also disclosed in “Methods in Enzymology” Vol. 135 Part B, p. 401, Academic Press, 1987. The polymer must be heated above its melting temperature, which is about 92° C., and dissolved in the presence of water. At or above that temperature, the polymer melts and the molten polymer is then solvated by water to form a solution. The polymer remains soluble in water as long as the temperature is above the polymer's gel point, which is about 43° C. At and below the gel point, the polymer phase separates and becomes a hydrogel that takes on whatever shape the solution was just before gelling. Additionally, as the agarose approaches its gel point, the viscosity of the solution becomes higher and higher as the hydrogel begins to form.
Traditionally, for polysaccharide beads, such as are used in chromatography media, the heated solution is kept above its gel point and it is stirred into an immiscible, heated fluid, such as mineral or vegetable oil, to form beads. The two-phased material (beads of agarose in the immiscible fluid) is then cooled and the beads are recovered. The beads themselves are diffusionally porous and can then be used as made for size exclusion chromatography. Additionally, they can be further processed by crosslinking, addition of various capture chemistries such as affinity chemistries or ligands, positive or negative charge, hydrophobicity or the like or combinations of crosslinking and chemistries to enhance their capture capabilities.
Agarose has been used extensively to form porous beads, where the target product and/or impurities travel into and back out of the pores in a diffusion-driven process.